Preheat oven to 450 degrees.
Place zucchini, summer squash, onion, mushrooms, and sweet pepper on a baking sheet; spray with olive oil spray.
Roast in oven for 10 minutes or until vegetables are tender. Season to taste with salt and pepper.
Arrange vegetables on top of bread; drizzle with salad dressing.
Top with shredded cheese. Place on unheated rack of a broiler pan. Broil 4" from heat until cheese melts.